1. Field of the Invention
The present invention relates to a composition for substrate materials and process for the same, as well as a heat conductive substrate and process for the same, particularly to a technology for gaining a heat conductive substrate preferably utilized in the field of power electronics.
2. Description of the Related Art
Accompanying with the increased performance or miniaturization of electronic equipment, it is desired for semiconductor devices or electronic parts to be designed more densely or to be improved in performance. It is also desired for circuit substrates on which semiconductor devices or electronic components are mounted to be designed more densely and more miniaturized. Therefore it has become important to design a circuit substrate taking account of heat radiation. The following is an example of a heat conductive substrate which is a circuit substrate improved in heat radiating performance. This heat conductive substrate has a configuration where the wiring pattern is formed through an insulator layer on one or two sides of a metal plate such as copper or aluminum. In the following, this heat conductive substrate is referred to as a insulated metal substrate.
On the other hand, as an example of a heat conductive substrate improved in the heat radiating performance there is a heat conductive substrate in which a copper plate is integrated with a ceramic substrate formed of alumina, aluminum nitride or the like. In the following such a heat conductive substrate is referred to as a metal-attached ceramic substrate.
As for a metal-based substrate, it is preferable to make the insulator layer thinner to secure a better heat radiation. To make the insulator layer thinner, however, leads to disadvantages such that it becomes more susceptible to the effect of the noise or it becomes more difficult to secure enough withstand voltage against insulation.
And metal attached ceramic substrate can only be utilized for high current because of the reason that a manufacturing cost is higher compared to that of a metal-based substrate, therefore it is general to use a metal-based substrate for other objects.
In this way, it is difficult to achieve metal-based substrates and metal-attached ceramic substrates which satisfy both functions and manufacturing costs.
Therefore, in recent years, the following manufacturing process of heat conducting substrates is proposed. First of all, a resin composition where an inorganic filler with heat conductivity is filled up in a thermoplastic resin is prepared. And by injection molding and integrating this resin composition and a lead frame, a heat conductive module-type heat conductive substrate is formed.
Though a heat conductive module type heat conductive substrate manufactured in this manner can be secured in the mechanical strength better than that of metal-attached ceramic substrate, there occurs a disadvantage that heat radiation is worse because it is difficult to fill up inorganic filler in high density. The following is the reason why it is difficult to fill up inorganic filler in high density. Too much amount of filler increases the melting viscosity dramatically and makes it difficult to carry out kneading and injection molding. And because the filler works as an abrasive, the aberration of the metal mold is significant.
Therefore, recently, a heat conductive substrate manufactured by forming resin composition filled up with inorganic filler with good heat radiation integrated with a lead frame is proposed, as disclosed in, for example, the Japanese unexamined patent publication H10 (1998)-173097.
This heat conductive substrate is manufactured with the method shown in FIG. 9. That is to say, a mixture slurry including at least some inorganic filler and thermosetting resin is prepared. By forming a film from this mixture slurry a green sheet 31 is manufactured. After drying the green sheet 31, as shown in FIG. 9A, the green sheet 31 and the lead frame 32 are overlapped. After that, the green sheet 31 is cured by heating and pressurizing to produce a heat conductive substrate 34 constructed from a heat conductive cured body 33 and the lead frame 32 attached and integrated together, as shown in FIG. 9B.
In the above mentioned conventional heat conductive substrate 34, however, the following disadvantages occur. First, the green sheet 31 cannot be arranged without forming space with the lead frame 32 with the complicated form and the form precision cannot be fully maintained.
Moreover, in order to control the sheet thickness while manufacturing sheets, it is necessary to control and adjust the viscosity of the mixture slurry and a film-forming device, and work for that is difficult.
In addition, the sheet cutting process such as die punching using a metal mold for the convenience of processing sheets into a desired shape is indispensable, and therefore the processes have increased to raise the manufacturing costs.
In addition, the sheets cut off from the necessary parts cannot be utilized and they become dispensable, so it is not only disadvantageous from the point of cost effectiveness but also the resource is not utilized effectively.